The invention relates generally to motion detectors and more particularly relates to a motion detector based on the Doppler principle which employs a microwave module for emitting a microwave signal into a room under surveillance containing at least two frequencies and detecting a phase difference in the signals which is proportional to, inter alia, the distance of an object moving in the room under surveillance.
Motion detectors that employ the Doppler principle have a weakness in that small objects, such as, for example, insects or raindrops, situated near the detector can be distinguished only with very great difficulty from further removed, larger objects. As a result, distinguishing distant humans from small domestic animals at a lesser distance and from insects positioned near the detector is made considerably more difficult or even made impossible. In addition, there is the problem that oscillating fixed objects, such as, for example, curtains or leaves moved by the wind or even oscillating fans cannot be distinguished from actual motion occurrences. Because of these disadvantages, such motion detectors have a relatively high false alarm rate and are therefore used virtually exclusively as dual detectors.
To avoid the disadvantages mentioned, U.S. Pat. No. 4,697,184 proposes emitting a microwave signal containing alternating frequencies and deriving from the signal reflected by an object in the room under surveillance first and second Doppler signals that have a phase difference proportional to the distance of the object. The phase difference is measured and an alarm signal is triggered if the phase difference exceeds a specified limit value for a certain duration. In this system, which uses a hardware differential amplifier to determine the phase difference, interferences resulting from multiple-path propagation, such as, for example, reflections from walls, the floor and the ceiling and noise are not significantly suppressed.
The present invention is therefore intended to provide a microwave motion detector in which the interferences mentioned, such as multi-path interference, are reliably suppressed. The present invention is also intended to provide enhanced detection parameters, such as target velocity, expected position and the like.
The above objects are achieved, according to the invention, in that the phase difference is determined by an integral transformation. Compared with other methods for measuring phase, the integral transformation has the advantage of superior noise and other interference suppression.
In accordance with the invention, a motion detector based on the Doppler principle includes a microwave module for emitting a microwave signal containing at least two frequencies into a room under surveillance and for receiving reflections of said microwave signal. An evaluation stage is operatively coupled to the microwave module. The evaluation stage receives that reflected signals and generates first and second Doppler signals that have a phase difference proportional to the distance of an object reflecting the microwave signal. The evaluation stage applies an integral transformation to the received reflected signal to determine the phase difference.
In one embodiment of the motion detector, the integral transformation is additionally used to determine the sign of the phase difference and/or the radial velocity of the object concerned relative to the detector.
In another embodiment of the motion detector, the integral transformation is additionally used to determine the signal strength of the received signal.
Yet another embodiment of the motion detector includes two channels that are disposed downstream of the output of the microwave module in which the signals are amplified,
filtered and fed to a corresponding analog/digital converter. In this case, the integral transformation for two signals can be performed by integrating the absolute values of the signals in the two channels, multiplying the signals, integrating the result of this multiplication and also dividing the resultant signal.
In a further embodiment of the motion detector the microwave module emits a microwave signal containing more than two frequencies. This embodiment has the advantage that emitting more than two frequencies resolves ambiguities, such as may occur in two-frequency operation. Such ambiguities may be due, for example, to very distant reflections that are suppressed in the motion detector described in U.S. Pat. No. 4,697,184 by relatively complex and expensive hardware.
Yet a further embodiment of the motion detector includes a number of channels that correspond to the number of frequencies that are disposed downstream of the output of the microwave module and in which the signals are amplified, filtered and each fed to an analog/digital converter. In this case, the integral transformation is carried out by a Fourier transformation, a fast Fourier transformation or a wavelet transformation.
In another embodiment of the motion detector the microwave module and the evaluation stage form part of a first detector of a dual detector that contains, in addition, a second detector, such as a passive infrared detector, and the signals of the first detector and those of the second detector are fed to a common processing stage in which the signals are combined. In a further embodiment of the motion detector, the result of the combination contains information about the quality of the object moving in the room under surveillance.